Ultrasound imaging has been in wide use in a variety of industries. In the typical application, a piezoelectric element is used for both excitation and detection of the ultrasound signal. In a noninvasive use, such as the medical application of prenatal imaging, a piezoelectric element is placed in intimate contact with the pregnant woman's body to be tested and the element is excited with an electrical pulse with a center frequency selected for optimum penetration to the area of interest. The images allow assessment of the condition of the developing fetus. Arrays of sensors are often used to simplify data acquisition, improve spatial resolution, and improve signal to noise. Such arrays may also allow creation of a structured ultrasound beam that may be steered and provide for three-dimensional imaging applications.
Cannula and laproscopic medical applications of ultrasound imaging place greater restraints on the size of the excitation and sensing elements. Structures where the excitation and sensing elements are integrated onto a single substrate are required. Designs using a continuous sinusoidal excitation may make use of the Doppler frequency shift of the reflected signal to measure flow velocities. One disadvantage of such designs is that the electrical signal required for excitation of the piezoelectric device is generally several orders of magnitude larger than the electrically measured reflected signal using a piezoelectric detector. The designs therefore require high excitation voltages and low signal voltages on the same micro device. Interference requires careful design considerations for isolation of signals. Additionally, signal to noise often limits the applicability of the device. In many cases, the proper operation of piezoelectric devices is precluded by electrical noise.
The depth to which a sample may be probed is dependent upon the sample's material composition, the frequency of the ultrasound device and the sensitivity of the detector. Designs are required that will allow a variety of materials to be incorporated into the sensing element for optimum operation over a wide range of frequencies. Designs are most often unique to the application. Even in a single application, the sensor design might vary dependent upon the material. In a medical imaging scenario, the density, and therefore response of bone material, is significantly different from muscle, fat or fluids. Designs are required that can image all such materials.
The integration of optical sensors into an ultrasound system is known. Optical sensors incorporating a Mach-Zehnder interferometer are used in oil and gas exploration and in underwater applications. These designs are composed primarily of two optical fibers that equally split the light from a single source.
The interferometer makes use of the change of the refractive index in one of the glass optical fibers when it is placed under mechanical stress during detection of an acoustic signal, and when the light from the two optical fibers is recombined. Due to the mechanical stress applied along one arm of the interferometer and the resultant induced refractive index change, a phase difference between the light of the two paths in the interferometer is generated that is proportional to the acoustic signal. In these applications, the size of the probe is often not a concern and very long sensors, often several meters, are used for increased sensitivity. Another advantage of such probes is that the sensor electronics may be located remotely from the sensor. The sensor that is in contact with the sample consists only of optical fiber. The sensor may therefore be used in corrosive or otherwise dangerous environments. Additionally, since this is an optical sensor, it would not be affected by intrinsic or extrinsic electromagnetic interference, or noise inherent in the materials of typical sensors.
Each of the designs and applications make use of particular features of an ultrasound detector. There is, however, no single design that can incorporate the multiple advantages. There is a need for a small fully integrated ultrasound detector that can operate over a wide frequency range and that can be configured for a variety of applications. There is a need for a detector that can operate in both a continuous and a pulsed mode and that can be fabricated either as a single element or as an array of elements. There is a need for an ultrasound detector that eliminates the requirement to isolate the high voltages of an excitation piezoelectric device from the low level reflected signal. There is a need for an ultrasound device design that can operate in a variety of environments. There is a need for an ultrasound detector device that can be used for imaging applications, create structure beams for three dimensional imaging, and also be used for Doppler measurements. There is a need for an ultrasound device that can place the processing electronics either remotely from the sensor device or locally to the sensor device. There is a need for an ultrasound detector device that can incorporate excitation and detection on a single substrate but that can also be designed with excitation remote from detection.